Decoupled Injection Moulding Setup Principles for Technical Parts
Decoupled injection moulding decouples volumetric filling velocity from packing pressure to achieve tight tolerances and repeatable technical part production.
Dynamic Shear Rate
Meaning
High-speed melt flow through runner channels and narrow gates generates localized velocity gradients across the polymer stream. Mould design software calculates dynamic shear rate to predict viscous dissipation and polymer chain alignment inside the cavity. Polymer resins exhibit non-Newtonian shear-thinning behavior, where apparent viscosity drops dramatically as flow velocity increases.
Datasheet viscosity curves measured at low shear rates fail to reflect actual flow resistance during rapid injection phases.
Viscous Dissipation
Frictional heating within the flowing polymer melt increases local temperature when fluid layers slide rapidly past one another. Excessive dynamic shear rate induces localized thermal degradation, breaking polymer chains and discolouring sensitive resins. Polycarbonate and polyamide compounds suffer severe molecular weight reduction under excessive shear stress.
Processors optimize fill time to maintain shear rates within the safe processing window specified by resin suppliers.
Gate Geometry
Submarine gates and hot runner tips restrict melt passage, causing sharp spikes in fluid shear during initial cavity filling. Adjusting gate depth modifies the dynamic shear rate without requiring changes to overall machine injection speed setups.
Shear Degradation
Degraded resin fragments cause silver streaks and reduced impact strength in finished moulded parts. Operating below critical dynamic shear rate limits preserves material toughness and maintains consistent mechanical performance.